If I have an unpublished program that I have already made, can I enter it in NANY?  Should I start a new thread for it?

Yes.  Firefox will allow that.  But it doesn't make any difference what that is if you run multiple instances.  Firefox's setting only determines which profile gets loaded if you don't choose one.  It doesn't set which instance is the system's default Firefox browser.  That's the problem.  If I could do this using Firefox's own settings, I'd have done it already.  This is going to involve a trick of some sort that isn't there yet.  And I don't blame you if you don't want to get into it!  I just thought I would ask, because it's something that folks, I'm sure, would want and use, but doesn't exist yet.

Hello again, Mouser.  Have you thought about this any?

I run two instances of Firefox, and Firefox is the default browser on my system.  But only the first-launched instance will ever accept to open links from outside programs (such as thunderbird).  So, if I close that instance, then I no longer have any default browser open, even though the second instance is still running.   

This is a real pain.  If browser-tray-switch could solve this, I would jump up and down for joy!